>>941327If it's a big or well-organized company, the kind with a website detailing its operations, just contact HR and tell them you've been hired and need more info on completing the process.
If it's a small business, franchise, or less formal environment (like an auto shop or mcdonald's), I'd do the following (1) call the place (2) when the person picks up, as if he's the manager (3) if yes hang up (4) if no, ask them the manager's name. Tell them you met with him about something and can't remember his name. They won't think twice about giving it to you.
If neither of those applies, give more details. What kind of job, how did you contact this guy to begin with?